The present invention relates to transformers which provide isolation between two electrical circuits; and more particularly to transformers which isolate medical equipment connected to a patient from a source of relatively high voltage that powers the equipment.
Many types of medical equipment, such as temperature monitors, electrocardiograms, oximeters, or invasive blood pressure monitors, include sensors which are in contact with the patient. Although the sensors operate at relatively low voltage and current levels that do not present a shock hazard to the patient, that hazard can occur if high voltage from a supply line powering the equipment is applied to the sensor due to failure of the equipment's internal power supply. As a consequence, the U.S. Food and Drug Administration, which regulates medical equipment, has prescribed rules specifying the degree of electrical isolation required between components that contact the patient and the electrical supply lines.
The isolation requirement typically is satisfied by supplying power through an transformer. An exemplary medical apparatus 10, schematically illustrated in FIG. 1, has a non-isolated section 12 containing a power supply driver 14 which is connected via plug 16 to 120 or 240 volt alternating current supply lines in a building. The apparatus 10 also has circuitry in an isolated section 18 which includes a DC power supply 20 and components that sense and process biological function data. For example, a bio-amplifier 22 is connected by a cable 24 to sensors on medical patient 26. The processing components, such as bio-amplifier 22, may supply signals to other circuits and display devices in the non-isolated section 12 via optically isolated conductors (not shown).
The electricity for the isolated section 18 is supplied from driver 14 through an isolation transformer 28 to the DC power supply 20. The U.S. Food and Drug Administration has specified that the transformer, as well as other components which form the isolation barrier between the two apparatus sections 12 and 18 must be able to withstand a four kilovolt AC breakdown voltage with leakage current of ten microamperes or less when the supply line voltage is applied across the isolation barrier.
In common medical apparatus, the driver applies a high frequency excitation signal of 100-500 kHz to the transformer primary winding and harmonics of the excitation signal may be in the radio frequency spectrum. Other regulations specify the amount of electromagnetic interference (EMI) which the medical apparatus may radiate in the radio frequency (RF) spectrum. Those regulations also affect the design of the isolated power supply when the driver 14 applies a high frequency excitation signal to the primary winding of the transformer 28.
FIGS. 2 and 3 portray a typical isolation transformer utilized in previous medical equipment. The transformer comprises a solid ferrite core 30 with a central leg 32 passing through the aperture in an annular bobbin 34. The primary and secondary windings 36 and 38 are wound around the bobbin 34 and thus around the center leg 32 of the core 30. As shown in FIG. 3, the primary winding 36 comprises two layers of turns wound on the bobbin adjacent the center leg 32 of the transformer core with plastic tape 40 therebetween. Additional plastic tape 41 is wrapped around the outer periphery of the primary winding 36. The secondary winding 38 also is formed by two layers of turns with plastic tape 42 there between. The earth ground 17 (FIG. 1) is connected to the first turn 43 of the primary winding 36 and the isolated ground 23 in the isolated section 18 is connected to the first turn 44 of the secondary winding 38. For ease of illustration, the primary and secondary windings are being shown with each one having only two layers of turns. Normally this type of transformer has many more layers in each winding which further increases the problem described hereafter.
The instantaneous voltage per turn (Vturn) is the same for the primary and the secondary windings 36 and 38. In this arrangement, parasitic capacitances exist between adjacent turns of the second layer in primary winding 36 and the first layer within secondary winding 38. At any instant in time, the first turn 46 of the second layer in primary winding 36 has a potential n*(Vturn) with reference to the earth, or non-isolated, ground where n is the number of turns in one layer (e.g. nine). The second turn in the primary winding second layer has a potential given by [n*(Vturn)]+Vturn, the third turn potential is [n*(Vturn)]+[2*(Vturn)], and so on. The numbers within each circular representation of a conductor in FIG. 2 designate the voltage in each turn when Vturn equals one. The first turn 44 of the first layer in secondary winding 38 at the same instant in time has a potential of zero volts with reference to the isolated ground. The second turn in this layer of the secondary winding has a potential of Vturn, the third winding turn has a potential of 2*(Vturn), and so on. As is apparent, at any instant in time, there is a difference of potential of n*(Vturn) between adjacent turns in abutting layers of the primary and secondary windings.
As a result of this transformer configuration, a voltage Vgg is generated between the non-isolated and isolated grounds and is applied to the sensor cable 24. As a consequence a pulsatile current, due to high frequency excitation of the parasitic transformer capacitance, flows through the cable 24, patient 26 and then to earth ground through additional parasitic capacitance 27 between the patient and earth ground. The pulsatile current also is partially radiated as electromagnetic energy. This current then returns to the non-isolated section 12 through the earth ground connection 17.
Several problems are associated with this parasitic high frequency current. Such current flowing from the patient 26 to ground can mislead front end sensor signal processing components in the medical apparatus. In addition, common mode current flowing through sensitive electronics in the isolated section 18 can generate voltage drops on the ground conductor which also mislead amplifiers and other components. Further, the parasitic high frequency current may result in excessive electromagnetic interference because the apparatus acts as a RF transmitter with a resonant frequency defined by the length of the patient cable 24 and the sensor probes.
A possible solution to these problems involves bypassing the common mode voltage source presented by the transformer with a capacitor Cbp, designated 29 in FIG. 1. Ignoring impedance due to parasitic patient capacitance 27 and the radiated energy, the voltage Vgg between the two grounds is defined as: ##EQU1## where Zbp equals 1/(.omega.Cbp) and is the impedance of the bypass capacitor at excitation frequency .omega., Zcomm is the impedance of the transformer's parasitic capacitance at the excitation frequency, and Vcm=n*(Vturn). Hence: ##EQU2##
This relationship indicates that in order to reduce the inter-ground voltage Vgg, the bypass capacitance Cbp must be relatively large. However, as the bypass capacitance increases, the leakage current increases and often rises above acceptable limits. Therefore, another solution is dictated.